CMOS Push-Pull Power Amplifier With Even-Harmonic Cancellation

ABSTRACT

A power amplifier includes a push-pull pair of transistors including a first transistor inductively coupled to a voltage source and coupled to a ground, and a second transistor inductively coupled to the ground and coupled to the voltage source. Gates of the first and the second transistors are AC inputs configured to receive an AC signal having a fundamental frequency. Drain regions of the first and the second transistors are, respectively, first and second output nodes. The power amplifier further includes a capacitor coupled between the first output node and the second output node and where the capacitor is configured as a pathway for cancellation of even harmonic signals of the fundamental frequency of the AC signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of, and priority to, U.S. Provisional Patent App. No. 61/426,991, filed Dec. 23, 2010, titled “CMOS PUSH-PULL PA WITH EVEN HARMONIC CANCELLATION,” of Sutardja et al., which is incorporated herein by reference in its entirety for all purposes.

BACKGROUND

Particular embodiments of the present invention generally relate to power amplifiers. More specifically, particular embodiments of the present invention relate to a power amplifier configured to cancel even harmonic signals.

Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.

Power amplifiers are configured to amplify the power of a received AC signal, such as an RF signal. Traditional power amplifiers often include a single nMOS transistor or a cascode both with tank loading. FIG. 1 is a simplified schematic of a traditional power amplifier 100 that includes an nMOS transistor 105 coupled between an inductor 110 and ground. Inductor 110 may be coupled to a voltage source Vdd. Inductor 110 may be coupled to a capacitor 115 in a tank configuration for tuning the resonance of power amplifier 100. The nMOS transistor 105 may be in a common source configuration with the gate of the nMOS transistor 105 configured to operate as an input to receive an AC signal and the drain coupled to the output Vout of power amplifier 100. FIG. 2 is a simplified schematic of another traditional power amplifier 200 that may include first and second nMOS transistors 205 and 210 in series between an inductor 215 and ground. Inductor 215 may be coupled to a voltage source Vdd. Inductor 215 may also be coupled to a capacitor 220 in a tank configuration for tuning the resonance of power amplifier 200. The nMOS transistors 205 and 210 may be in a common source, common gate configuration with the gate of nMOS transistor 205 configured to operate as an input to receive an AC signal and a drain of transistor 210 coupled to the output of power amplifier 200.

In traditional power amplifiers, such as power amplifier 100 and 200 described above, there are typically a number of nonlinear components at the drain of the transistor adjacent to the inductor. For example, the even harmonics, and especially the 2^(nd) harmonic, of a received AC signal tend to be fairly large at the drain of the transistor adjacent to the inductor. The averaging DC current from the even harmonics tends to be relatively large and flows in the inductor generating a relatively large amount of heat. As a result of the heat in the inductor, the inductance may change and in a worst case scenario the inductor may catastrophically fail.

Therefore, it would be desirable to provide new power amplifiers that are configured to reduce the nonlinear components and reduce the adverse effects of the nonlinear components in power amplifiers.

SUMMARY

Particular embodiments of the present invention generally relate to power amplifiers. More specifically, particular embodiments of the present invention relate to a power amplifier configured to cancel even harmonic signals.

According to one specific embodiment, a power amplifier includes a push-pull pair of transistors including a first transistor inductively coupled to a voltage source and coupled to a ground. A second transistor is inductively coupled to the ground and is coupled to the voltage source. Gates of the first and the second transistors are AC inputs configured to receive an AC signal having a fundamental frequency. Drain regions of the first and the second transistors are, respectively, first and second output nodes. The power amplifier further includes a capacitor coupled between the first output node and the second output node where the capacitor is configured as a pathway for cancellation of even harmonic signals of the fundamental frequency of the AC signal.

According to another specific embodiment, the power amplifier further includes a first inductor disposed between the first transistor and the voltage source. The first output node is between the first transistor and the first inductor. The power amplifier further includes a second inductor disposed between second transistor and the ground. The second output node is between the second transistor and the second inductor.

According to another specific embodiment, the first output node is between the drain of the first transistor and the first inductor, and the second output node is between the drain of the second transistor and the second inductor.

According to another specific embodiment, a source of the first transistor is coupled to the ground, and a source of the second transistor is coupled to the voltage source.

According to another specific embodiment, the first transistor and the second transistor are in a common source configuration.

According to another specific embodiment, the power amplifier further includes a first tank capacitor coupled in parallel with the first inductor and configured to tune the resonant frequency of a first inductor. The power amplifier further includes a second tank capacitor coupled in parallel with the second inductor and configured to tune the resonant frequency of a second inductor.

According to another specific embodiment, the capacitor is substantially not a pathway for cancellation of fundamental frequency of the AC signal.

According to another specific embodiment, the first output node and the second output node are the same output of the power amplifier and are configured to be combined by a combiner.

According to another embodiment, a power amplifier includes a first transistor, and a first inductor coupled in series with the first transistor between a voltage source and a ground. A first node between the first transistor and the first inductor is a first output. The power amplifier further includes a second transistor and a second inductor coupled in series with the second transistor between the voltage source and the ground. A second node between the second transistor and the second inductor is a second output. Gates of the first and the second transistors are AC inputs configured to receive an AC signal having a fundamental frequency. The power amplifier further includes a capacitor coupled between the first node and the second node and is configured as a pathway for cancellation of even harmonic signals of the fundamental frequency of the AC signal.

According to another embodiment, a fully-differential power amplifier includes first, second, third, and fourth transistor-inductor pairs each coupled in series between a voltage source and a ground. The fully-differential power amplifier further includes a first capacitor coupled between a first node, which is between the first transistor-inductor pair, and a second node, which is between the second transistor-inductor pair. The fully-differential power amplifier further includes a second capacitor coupled between a third node, which is between the third transistor-inductor pair, and a fourth node, which is between the fourth transistor-inductor pair. A first transistor of the first transistor-inductor pair is coupled to ground, and a first inductor of the first transistor-inductor pair is coupled to a voltage source. A second transistor of the second transistor-inductor pair is coupled to the voltage source, and a second inductor of the second transistor-inductor pair is coupled to the ground. A third transistor of the third transistor-inductor pair is coupled to ground, and a third inductor of the third transistor-inductor pair is coupled to a voltage source. A fourth transistor of the fourth transistor-inductor pair is coupled to the voltage source, and a fourth inductor of the fourth transistor-inductor pair is coupled to the ground. Gates of the first and the second transistors are plus input configured to receive an AC signal having a fundamental frequency, and gates of the third and the fourth transistors are minus input configured to receive the AC signal having the fundamental frequency. The first and second capacitors are configured to cancel even harmonics of the fundamental frequency of the AC signal.

According to one specific embodiment of the fully-differential power amplifier, the first and the third transistor are a first differential pair, and the second and fourth transistors are a second differential pair. Further, the first, second, third, and fourth transistors may be in a common source configuration.

According to another specific embodiment of the fully-differential power amplifier, the fully-differential power amplifier further includes a combiner having first, second, third, and fourth combiner inductors respectively in series. The first inductor and the first combiner inductor are inductively coupled. The third inductor and the second combiner inductor are inductively coupled. The fourth inductor and the third combiner inductor are inductively coupled. The second inductor and the fourth combiner inductor are inductively coupled.

According to another specific embodiment of the fully-differential power amplifier, the fully-differential power amplifier further includes a combiner having first and second combiner inductors in series. The first inductor, the second inductor, and the first combiner inductor are inductively coupled. The third inductor, the fourth inductor, and the second combiner inductor are inductively coupled.

The following detailed description and accompanying drawings provide a more detailed understanding of the nature and advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic of a traditional power amplifier that includes an nMOS transistor coupled in series with an inductor between a voltage source and ground;

FIG. 2 is a simplified schematic of another traditional power amplifier that may include first and second nMOS transistors coupled in series with an inductor between a voltage source and ground;

FIG. 3 is a simplified schematic of a power amplifier according to one embodiment of the present invention;

FIG. 4A is a simplified schematic of a fully-differential power amplifier according to one embodiment of the present invention;

FIG. 4B is an alternative simplified schematic of the fully-differential power amplifier shown in FIG. 4A; and

FIG. 5 is a simplified schematic of a fully-differential power amplifier according to an alternative embodiment.

DETAILED DESCRIPTION

Described herein are embodiments of a power amplifier and a method of operation for the power amplifier where the power amplifier is configured to cancel even harmonic signals present in the power amplifier.

In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. Particular embodiments as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein.

Power amplifiers are configured to amplify the power of a received AC signal, such as an RF signal, for subsequent transmission of the amplified AC signal. Power amplifiers may be included in a variety of mobile devices, such as mobile telephones. A power-amplified AC signal may be directed through an antenna of a mobile device for transmission.

FIG. 3 is a simplified schematic of a power amplifier 300 according to one embodiment of the present invention. Power amplifier 300 includes a push-pull pair of transistors 305 and 310, (referred to as transistors 305 and 310). Power amplifier 300 further includes first and second inductors 315 and 320 and a capacitor 325. Power amplifier 300 may also include first and second tank capacitors 330 and 335. Transistors 305 and 310 may be metal-oxide-semiconductor field-effect transistors (MOSFETs), bipolar-junction transistors (BJTs), or other transistor types. For convenience, power amplifier embodiments are described herein as including MOSFETs. While power amplifier embodiments are described herein as including MOSFETs, it will be understood by those of skill in the art that BJTs or other types of transistors may be appropriately substituted for the MOSFETs, and these power amplifier embodiments that include BJTs or the like are considered to be within the scope and purview of the power amplifier embodiments of the present invention.

According to one embodiment, transistor 305 is an nMOS transistor with a drain 305 a (sometimes referred to a drain region) coupled to a first end of inductor 315 where a second end of inductor 315 is coupled to a voltage source Vdd. A source 305 b of transistor 305 may be coupled to ground. Specific configurations of the sources and the drains of transistors 305 and 310 are described herein for convenience of explanation of specific embodiments. Alternative embodiments of the power amplifiers may include alternative configurations of the sources and the drains of transistors 305 and 310 as will be understood by those of skill in the art. A gate 305 c of transistor 305 may be a first AC input 340 a configured to receive an AC signal.

Capacitor 330 is coupled to inductor 315 in parallel in a tank configuration. Capacitor 330 is configured to tune the resonant frequency of inductor 315.

According to one embodiment, transistor 310 is a pMOS transistor with a drain 310 a coupled to a first end of inductor 320 where a second end of inductor 320 is coupled to ground. A source 310 b of transistor 310 is coupled to the voltage source Vdd. A gate 310 c of transistor 310 may be a second AC input 340 b configured to receive the AC signal supplied to the first AC input 340 a. Capacitor 335 may be coupled to inductor 320 in parallel in a tank configuration. Capacitor 335 is configured to tune the resonant frequency of inductor 320. Inductors 315 and 320 may be considered the respective loads of the push-pull pair of transistors 305 and 310.

Power amplifier 300 includes a first output node 345 a disposed between inductor 315 and the drain 305 a of transistor 305. Power amplifier 300 further includes a second output node 345 b disposed between inductor 320 and the drain 310 a of transistor 310. According to one embodiment capacitor 325 is coupled between the first output node 345 a and the second output node 345 b.

The even harmonics (e.g., 2^(nd) harmonic, 4^(th) harmonic, etc.) of the AC signal that are at the drain of transistor 305 are generally 180 degrees out of phase with the even harmonics of the AC signal that are at the drain of transistor 310. The capacitance of capacitor 325 is determined such that the circuit path between the two output nodes is substantially a short circuit for the even harmonics, but is not a short circuit for the fundamental frequency of the AC signal. According to one specific embodiment, the capacitance of capacitor 325 is approximately 20 picofarads for an approximately 2 gigahertz AC signal. As the even harmonics on either side of capacitor 325 are out of 180 degrees phase and as capacitor 325 is substantially a short for the even harmonics on either side of capacitor 325, the even harmonics on either side of capacitor 325 tend to cancel each other. Because the even harmonics on either side of capacitor 325 tend to cancel each other, capacitor 325 ensures that the even harmonics do not substantially pass into the inductors 315 and 320. As a result, unnecessary heating of the inductors 315 and 320 via the even harmonics is inhibited. Capacitor 325 may be replaced with alternative circuits that provide capacitance, such as a diode or the like according to one alternative embodiment.

The voltage at the first output node 345 a (i.e., the voltage at the drain of transistor 305) may be expressed as: V_(dn)=α_(0n)+α_(1n)v(f₀)+α_(2n)v/(2f₀)+α_(3n)v(3f₀)+ . . . , and the voltage at the second output node 345 b (i.e., the voltage at the drain of transistor 310) may be expressed as: V_(dp)=α_(0p)+α_(1p)v(f₀)+α_(2p)v(2f₀)+α_(3p)v(3f₀)+ . . . . The even coefficients of the voltage expressions for nMOS transistors and pMOS transistors have different polarities, providing for the substantial cancellation of the even harmonics across capacitor 325.

Power amplifier 300 is a single stage power amplifier that is not fully differential. Two power amplifiers 300 may be inductively coupled to form a fully-differential power amplifier according to one embodiment of the present invention.

FIG. 4A is a simplified schematic of a fully-differential power amplifier 400 according to one embodiment of the present invention. Fully-differential power amplifier 400 includes a first power amplifier 400 a and a second power amplifier 400 b. Power amplifiers 400 a and 400 b have substantially the same configuration as power amplifier 300 described above. Power amplifiers 400 a and 400 b are both configured to cancel the even harmonics of the fundamental frequency of a received AC signal. The numbering scheme of the power amplifiers used with respect to fully-differential power amplifier 400 is changed for convenience as fully-differential power 400 includes first and second stages where each stage includes a power amplifier 300.

Power amplifier 400 a includes a push-pull pair of transistors 405 a and 410 a, which are referred to herein as transistors 405 a and 410 a. Power amplifier 400 a further includes first and second inductors 415 a and 420 a and a capacitor 425 a. Power amplifier 400 may also include first and second tank capacitors 430 a and 435 a. Transistors 405 a and 410 a are MOSFETs, BJTs, or other transistors types. According to one embodiment, transistor 405 a is an nMOS transistor, and transistor 410 a is a pMOS transistor. The electronic components of power amplifier 400 a are configured substantially the same as the corresponding electronic components of power amplifier 300. For example, transistor 405 a may be in a common source configuration and coupled in series with inductor 415 a. The series pair of transistor 405 a and inductor 415 a are coupled between the voltage source Vdd and the ground where transistor 405 a is coupled to the ground and inductor 415 a is coupled to the voltage source Vdd. Transistor 410 a is similarly in a common source configuration and is coupled in series with inductor 420 a. The series pair of transistor 410 a and inductor 420 a are coupled between the voltage source Vdd and the ground with transistor 410 a coupled to the voltage source Vdd and inductor 420 a coupled to the ground.

Power amplifier 400 b includes a push-pull pair of transistors 405 b and 410 b, which are referred to herein as transistors 405 b and 410 b. Power amplifier 400 b further includes first and second inductors 415 b and 420 b and a capacitor 425 b. Power amplifier 400 may also include first and second tank capacitors 430 b and 435 b. Transistors 405 b and 410 b may be MOSFETs, BJTs, or other transistor types. According to one embodiment, transistor 405 b is an nMOS transistor, and transistor 410 b is a pMOS transistor. The electronic components of power amplifier 400 a are configured substantially the same as the corresponding electronic components of power amplifier 300. For example, transistor 405 b may be in a common source configuration and coupled in series with inductor 415 b. The series pair of transistor 405 b and inductor 415 b are coupled between the voltage source Vdd and the ground where transistor 405 b is coupled to the ground and inductor 415 b is coupled to the voltage source Vdd. Transistor 410 b is similarly in a common source configuration and is coupled in series with inductor 420 b. The series pair of transistor 410 b and inductor 420 b are between the voltage source Vdd and the ground with transistor 410 b coupled to the voltage source Vdd and inductor 420 b coupled to the ground.

The first and second output nodes of power amplifier 400 a are coupled by capacitor 425 a. The first and second output nodes of power amplifier 400 b are coupled by capacitor 425 b. Capacitor 425 a is configured to provide a circuit path for the cancellation of even harmonics on opposite sides of capacitor 425 a. Similarly, capacitor 425 b is configured to provide a circuit path for the cancellation of even harmonics on opposite sides of capacitor 425 b.

The gates of transistors 405 a and 410 a are the AC inputs for power amplifier 400 a and are the “plus” inputs of fully-differential power amplifier 400. The gates of transistors 405 b and 410 b are the AC inputs for power amplifier 400 b and are the “minus” inputs of fully-differential power amplifier 400. The plus inputs of the fully-differential power amplifier are designated with “+” symbols in FIG. 4A, and the minus inputs are designated with “−” symbols in FIG. 4A. The nMOS transistors 405 a and 405 b are a fully differential n-pair and the pMOS transistors 410 a and 410 b are a fully differential p-pair.

According to one embodiment of the present invention, a combiner 460 is configured to combine the outputs of fully-differential power amplifier 400 to deliver an amplified AC signal to an antenna 465 or the like. Combiner 460 includes a first, second, third, and fourth inductors 470 a, 470 b, 470 c, and 470 d coupled together in series and coupled to antenna 465. Inductors 470 a, 470 b, 470 c, and 470 d are inductively coupled, respectively, to inductors 415 a, 420 a, 415 b, and 420 b where each pair of inductively coupled inductors is a transformer.

FIG. 4B is an alternative simplified schematic of the fully-differential power amplifier 400 shown in FIG. 4A. Transistors 405 a and 405 b are shown as a driver 405 with positive and negative differential outputs, and transistors 410 a and 410 b are shown as a driver 410 also with positive and negative differential outputs. The fully-differential power amplifier 400 as shown in FIG. 4B clearly shows the serial nature of the first, the second, the third, and the fourth inductors 470 a, 470 b, 470 c, and 470 d. As shown in FIG. 4B, the laterally adjacent inductors 415 a and 470 a are inductively coupled, the laterally adjacent inductors 415 b and 470 b are inductively coupled, the laterally adjacent inductors 420 a and 470 d are inductively coupled, and the laterally adjacent inductors 420 b and 470 c are inductively coupled. The node between inductors 415 a and 415 b may be coupled to a reference voltage, such as Vdd. The node between inductors 420 a and 420 b may be coupled to ground.

FIG. 5 is a simplified schematic of a fully-differential power amplifier 500 according to an alternative embodiment. The same reference numeral schema used for the foregoing described figures is used in FIG. 5 to identify the same elements or substantially similar elements. Fully-differential power amplifier 500 is substantially similar to fully-differential power amplifier 400 but differs in that a combiner 480 of fully-differential power amplifier 500 differs from combiner 460 of fully-differential power amplifier 400. Combiner 480 includes the first inductor 470 a inductively coupled to both inductors 415 a and 420 a, and the second inductor 470 b inductively coupled to both inductors 415 b and 420 b. Inductors 470 a and 470 b are disposed in series. Inductors 470 a, 415 a, and 420 a are substantially parallel, and inductors 470 b, 415 b, and 420 b are also substantially parallel. Combiner 480 then applies the outputs onto antenna 465.

The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations, and equivalents may be employed without departing from the scope of the invention as defined by the claims. 

1. A power amplifier comprising: a push-pull pair of transistors including a first transistor inductively coupled to a voltage source and coupled to a ground, and a second transistor inductively coupled to the ground and coupled to the voltage source, wherein: gates of the first and the second transistors are AC inputs configured to receive an AC signal having a fundamental frequency, and drain regions of the first and the second transistors are, respectively, first and second output nodes; and a capacitor coupled between the first output node and the second output node and configured as a pathway for cancellation of even harmonic signals of the fundamental frequency of the AC signal.
 2. The power amplifier of claim 1, further comprising: a first inductor disposed between the first transistor and the voltage source, wherein the first output node is between the first transistor and the first inductor; and a second inductor disposed between second transistor and the ground, wherein the second output node is between the second transistor and the second inductor.
 3. The power amplifier of claim 2, wherein the first output node is between the drain of the first transistor and the first inductor, and the second output node is between the drain of the second transistor and the second inductor.
 4. The power amplifier of claim 2, wherein a source of the first transistor is coupled to the ground, and a source of the second transistor is coupled to the voltage source.
 5. The power amplifier of claim 1, wherein the first transistor and the second transistor are in a common source configuration.
 6. The power amplifier of claim 2, further comprising: a first tank capacitor coupled in parallel with the first inductor and configured to tune a resonant frequency of the first inductor; and a second tank capacitor coupled in parallel with the second inductor and configured to tune a resonant frequency of the second inductor.
 7. The power amplifier of claim 1, wherein the capacitor is substantially not a pathway for cancellation of fundamental frequency of the AC signal.
 8. The power amplifier of claim 1, wherein the first output node and the second output node are the same output of the power amplifier and are configured to be combined by a combiner.
 9. A power amplifier comprising: a first transistor; a first inductor coupled in series with the first transistor between a voltage source and a ground, wherein a first node between the first transistor and the first inductor is a first output; a second transistor; a second inductor coupled in series with the second transistor between the voltage source and the ground, wherein a second node between the second transistor and the second inductor is a second output, and gates of the first and the second transistors are AC inputs configured to receive an AC signal having a fundamental frequency; and a capacitor coupled between the first node and the second node and configured as a pathway for cancellation of even harmonic signals of the fundamental frequency of the AC signal.
 10. The power amplifier of claim 9, wherein: the first transistor and the second inductor are coupled to the ground, and the second transistor and the first inductor are coupled to the voltage source.
 11. The power amplifier of claim 10, wherein a source of the first transistor is coupled to ground and a source of the second transistor is coupled to the voltage source.
 12. The power amplifier of claim 11, wherein a drain of the first transistor is coupled to the first inductor, and a drain of the second transistor is coupled to the second inductor.
 13. The power amplifier of claim 12, wherein the first node is between the drain of the first transistor and the first inductor, and the second node is between the drain of the second transistor and the second inductor.
 14. The power amplifier of claim 9, further comprising: a first tank capacitor coupled in parallel with the first inductor and configured to tune a resonant frequency of the first inductor; and a second tank capacitor coupled in parallel with the second inductor and configured to tune a resonant frequency of the second inductor.
 15. The power amplifier of claim 9, wherein the first transistor and the second transistor are in a common source configuration.
 16. The power amplifier of claim 9, wherein the capacitor is substantially not a pathway for cancellation of the AC signal.
 17. The power amplifier of claim 9, wherein the second output and the first output are the same output of the power amplifier and are configured to be combined by a combiner.
 18. A fully-differential power amplifier comprising: first, second, third, and fourth transistor-inductor pairs each coupled in series between a voltage source and a ground; a first capacitor coupled between a first node, which is between the first transistor-inductor pair, and a second node, which is between the second transistor-inductor pair; and a second capacitor coupled between a third node, which is between the third transistor-inductor pair, and a fourth node, which is between the fourth transistor-inductor pair; wherein: a first transistor of the first transistor-inductor pair is coupled to ground, and a first inductor of the first transistor-inductor pair is coupled to a voltage source, a second transistor of the second transistor-inductor pair is coupled to the voltage source, and a second inductor of the second transistor-inductor pair is coupled to the ground, a third transistor of the third transistor-inductor pair is coupled to ground, and a third inductor of the third transistor-inductor pair is coupled to a voltage source, a fourth transistor of the fourth transistor-inductor pair is coupled to the voltage source, and a fourth inductor of the fourth transistor-inductor pair is coupled to the ground, gates of the first and second transistors are plus inputs configured to receive an AC signal having a fundamental frequency, and gates of the third and fourth transistors are minus inputs configured to receive the AC signal having the fundamental frequency, and the first and second capacitors are configured to cancel even harmonics of the fundamental frequency of the AC signal.
 19. The fully-differential power amplifier of claim 18, wherein: the first and the third transistors are a first differential pair, and the second and fourth transistors are a second differential pair.
 20. The fully-differential power amplifier of claim 18, wherein the first, second, third, and fourth transistors are in a common source configuration.
 21. The fully-differential power amplifier of claim 18, further comprising a combiner having first, second, third, and fourth combiner inductors respectively in series, wherein: the first inductor and the first combiner inductor are inductively coupled, the third inductor and the second combiner inductor are inductively coupled, the fourth inductor and the third combiner inductor are inductively coupled, and the second inductor and the fourth combiner inductor are inductively coupled.
 22. The fully-differential power amplifier of claim 18, further comprising a combiner having first and second combiner inductors in series, wherein: the first inductor, the second inductor, and the first combiner inductor are inductively coupled, and the third inductor, the fourth inductor, and the second combiner inductor are inductively coupled. 